Prince William and Kate Middleton hosted 27 members of their staff on Tuesday Dec. 18 for a Christmas party and lunch at the Bumpkin restaurant in London's Notting Hill neighborhood, a source confirms to Us Weekly.

Celebrating with members of the Household Cavalry, their press team, private secretaries and other staffers present, Kate and William, both 30, dined at a private room at the eatery -- right beneath a portrait of Her Majesty, Queen Elizabeth II!

Clad in a floral blue and white dress with a dinner jacket, the pregnant Duchess "sipped soft drinks," the source says, adding that her "health is improving" after dealing with acute morning sickness earlier this month.

And, as is usually the case with the spouses, the private bash wasn't extravagant, with everyone eating from the price-fixe Christmas Turkey menu, which cost a humble 30 pounds, or approximately $48, a head.

It's the second known outing in the past few days for the pregnant Duchess, who looked radiant and still quite slim in a green Alexander McQueen gown at the BBC Sports Personality of the Year ceremony Dec. 16 -- telling former soccer player Gary Lineker that she's feeling "very well."